Active implantable medical devices that collect (sense) and deliver (stimulate) signals in the atrium as well as the ventricle are subject to a disadvantage known as a "cross-talk" or "cross-sensing" phenomenon. This phenomenon occurs when, in the atrial cavity, the pacemaker detects a signal coming from a depolarization following a preceding ventricular stimulation (this cross-sensing phenomenon will be designated in the following as "CTVA" ("Cross-Talk Ventriculo-Atrial")). The device can wrongly interpret this signal as resulting from a spontaneous depolarization of the atrium, with the result that this detected phenomenon is prejudicial to the expected functioning of the device.
In current devices, this CTVA phenomenon is all the more emphasized because of the relatively greater the sensitivity of input amplifiers of the atrial stage; this high sensitivity is used to be able to sense and interpret signals of low amplitude in the atrium, in case of TdRA (Troubles of the Atrial Rhythm), a generic term that covers various atrial arrhythmia such as tachycardia, fibrillation, flutter, etc., which troubles are all characterized by a rapid, abnormal atrial rhythm at the detection.
To improve both the sensitivity and the selectivity of the atrial sensing and detection, it is therefore necessary to remedy the consequences of this cross-sensing.
A first solution concerns creating a time interval called "blanking", during which the atrial amplifier is completely disconnected from the sensing circuit during a ventricular stimulation (as described, for example, in U.S. Pat. No. 4,825,870 for an amplifier of signals detected in the ventricular cavity). But this blanking period is released (triggered) only during stimulation and has only a short duration; the former does not therefore, protect, against the delayed detection of the signal of depolarization.
EP-A-0 594 957 describes another manner to process the problem of the CTVA. It proposes to analyze collected signals and, when a signal in the atrium is detected at the same time that a signal is present on the ventricular stage, the device deduces from this concomitance that the detected atrial signal has its origin in the ventricle. Proceeding in this manner, however, suffers the disadvantage that one cannot detect spontaneous signals emitted by two cardiac cavities that are totally desynchronized. Further, such a processing of signals considers only the spontaneous events, and not the stimulated events.
Another solution would be to increase the atrial refractory period. This solution, although easy to implement in current pacemakers, would have the disadvantage to decrease the atrial sensing window. The rapid atrial arrythmias could then be partially detected, limiting the efficiency of the diagnosis algorithms.
Further, EP-A-0 705 620 describes a dynamic window technique so as to better recognize signals coming from a tachycardia, and to distinguish them from signals coming from a CTVA.